Typically, aircraft will approach an airport under the guidance of air traffic controllers. The air traffic controllers are tasked with ensuring the safe arrival of aircraft at their destination, while also ensuring the capacity of the airport is maximised. The former requirement is generally met by ensuring minimum specified separations are maintained between aircraft. Air traffic control is subject to uncertainties that may act to erode the separation between aircraft such as variable winds, both in speed and direction, and different piloting practices. Nonetheless, large numbers of aircraft can operate safely confined in a relatively small space since air traffic control can correct for these uncertainties at a tactical level using radar vectoring, velocity change and/or altitude change. As a result, a typical approach to an airport will involve a stepped approach where the aircraft is cleared to descend in steps to successively lower altitudes as other air traffic allows.
Air traffic noise around airports has important social, political and economical consequences for airport authorities, airlines and communities. An affordable way to tackle the noise problem in the vicinity of airports is to develop new navigation procedures that reduce the number of aircraft that fly over sensitive areas at low altitude with high thrust settings and/or with non-clean aerodynamic configurations (e.g. with landing gear and/or flaps deployed). Unfortunately, conventional step-down approaches act to exacerbate this problem as aircraft are held at low altitudes, where engine thrust must be sufficient to maintain level flight.
Continuous descent approaches (CDAs) are well known. These approaches see the aircraft approach an airport by descending continuously with the engines set to idle or close to idle. U.S. Pat. No. 4,825,374 describes a typical continuous descent approach implementation where an aircraft operates during the approach using a control law that maintains a constant airspeed by altering the attitude of the aircraft.
Clearly, continuous descent approaches are highly beneficial in terms of noise as they ensure that aircraft are kept as high as possible above sensitive areas while at the same time reducing the noise production at the source through optimum use of the engine and flaps. Continuous descent approaches also benefit fuel efficiency and minimise flight time.
However, continuous descent approaches must be planned in detail before commencing the approach and cannot be subjected to tactical corrections to ensure safe aircraft separation like those used in conventional step-down approaches. This obliges air traffic controllers to impose large separations between aircraft to guarantee that the aircraft arrive at the airport separated by a safe distance, bearing in mind the potential differences in aircraft separation as a result of wind changes and other uncertainties. Such an increase in separation results in an undesirable reduction in airport capacity.
The capacity penalty associated with continuous descent approaches prevents their widespread use in airports and, to date, continuous descent approaches have only been used at airports with low levels of air traffic or at busier airports during quiet times (e.g. at night). Thus, it is desirable to have continuous descent approach procedures that minimise uncertainties in the position and ground speed histories of the aircraft, and as a result, in the aircraft arrival times. This would allow air traffic controllers to reduce safely the separation between aircraft, thus satisfying the capacity needs of modern airports.